What happened

Shares of Karyopharm Therapeutics (KPTI -4.72%), a precommercial biopharmaceutical company, are plummeting in response to briefing documents released by the Food and Drug Administration ahead of an advisory committee meeting. Investors expecting a rough road ahead have knocked the stock 46.7% lower as of 3:56 p.m. EST on Friday.

So what

Karyopharm Therapeutics convinced the FDA to grant a priority review for selinexor, but the agency insisted on an advisory committee meeting to discuss the application with independent experts in a public setting first. Shortly before each of these meetings the agency publishes a giant briefing document that, among other things, describes issues likely to impede an approval.

A group of people in suits looking down at a colleague sprawled prone on a sidewalk

Image source: Getty Images.

Investors threw Karyopharm stock out a window today because the issues highlighted by regulators are almost certain to result in a negative recommendation. The application for accelerated approval is based on results from the single-arm Storm trial, which combined selinexor with a high dose of standard chemo for patients with relapsed and refractory multiple myeloma.

During the 123-patient Storm study, 25.4% of patients showed a response to treatment with selinexor plus chemo, which isn't a whole lot better than you'd expect from this group when treated with the same dose of chemo on its own.

Now what

Although selinexor plus chemo elicited a response from extremely difficult-to-treat patients, one look at the safety data says this combo therapy isn't going anywhere without proof of a survival benefit first. The Storm study treated just 123 patients, but 10 died due to treatment-emergent adverse events.

High-dose chemotherapy is dangerous by itself, and combining it with selinexor doesn't appear to make it any safer. A whopping 93.5% of patients treated with the combination required hospitalization due to side effects.

Although selinexor probably isn't going to earn an accelerated approval to treat fourth-line multiple myeloma, Karyopharm probably won't need to tap investors for more cash until its lead candidate has another chance to prove itself. The company finished December with $330 million in cash, which should be enough to keep operations humming along until early 2020.

Check out the latest Karyopharm earnings call transcript.